1. Field of the Invention
The present invention relates to a process for treatment of cellulosic material, as for example, knitted or woven cotton fabric. More specifically, the invention relates to a process for enzymatic bioscouring of cellulosic material under alkaline conditions.
2. Description of the Related Art
The processing of cellulosic material, as for example cotton fiber, into a material ready for garment manufacture involves several steps: spinning of the fiber into a yarn; construction of woven or knit fabric from the yarn and subsequent preparation, dyeing and finishing operations. Woven goods are constructed by weaving a filling yarn between a series of warp yarns; the yarns could be two different types. Knitted goods are constructed by forming a network of interlocking loops from one continuous length of yarn. The preparation process prepares the textile for the proper response in dyeing operations. The sub-steps involved in preparation are desizing (for woven goods), scouring and bleaching. A one step combined scour/bleach process is also used in the industry.
The processing regime can be either batch or continuous with the fabric being contacted by the liquid processing stream in open width or rope form. Continuous operations generally use a saturator whereby chemicals are applied to the fabric, followed by a heated dwell chamber where the chemical reaction takes place. A washing section then prepares the fabric for the next processing step. Batch processing generally takes place in one processing bath whereby the fabric is circulated through the bath. After a reaction period, the chemicals are drained, fabric rinsed and the next chemical is applied. Discontinuous pad-batch processing involves a continuous application of processing chemical followed by a dwell period which in the case of cold pad-batch might be one or more days. Desizing. Woven goods are the prevalent form of textile fabric construction. The weaving process demands a "sizing" of the warp yarn to protect it from abrasion. Starch, polyvinyl alcohol, carboxymethyl cellulose, waxes and acrylic binders are examples of typical sizing chemicals used because of availability and cost. The size must be removed after the weaving process as the first step in preparing the woven goods.
The sized fabric in either rope or open width form is brought in contact with the processing liquid containing the desizing agents. The desizing agent employed depends upon the type of size to be removed. The most common sizing agent for cotton fabric is based upon starch. Therefore most often, woven cotton fabrics are desized by a combination of hot water, the enzyme alpha amylase and a wetting agent or surfactant. The cellulosic material is allowed to stand with the desizing chemicals for a "holding period" sufficiently long to accomplish the desizing. The holding period is dependent upon the type of processing regime and the temperature and can vary from 15 minutes to 2 hours, or in some cases, several days. Typically, the desizing chemicals are applied in a saturator bath which generally ranges from about 15.degree. C. to 60.degree. C. The fabric is then held in equipment such as a "J-box" which provides sufficient heat, usually between 50.degree. C. to 100.degree. C. to enhance the activity of the desizing agents. The chemicals, including the removed sizing agents, are washed away from the fabric after the termination of the holding period.
In order to ensure a high whiteness and/or a good dyeability, the size and other applied must be thoroughly removed, and it is generally believed that an efficient desizing is of crucial importance to the following preparation processes: scouring and bleaching.
Scouring. The scouring process removes much of the non-cellulosic compounds naturally found in cotton. In addition to the natural non-cellulosic impurities, scouring can remove residual manufacturing introduced materials such as spinning, coning or slashing lubricants. The scouring process employs sodium hydroxide or related causticizing agents such as sodium carbonate, potassium hydroxide or mixtures thereof. Generally an alkali stable surfactant is added to the process to enhance solubilization of hydrophobic compounds and/or prevent their redeposition back on the fabric. The treatment is generally at a high temperature, 80.degree. C.-100.degree. C., employing strongly alkaline solutions of the scouring agent, e.g., pH 13-14. Due to the non-specific nature of chemical processes not only are the impurities but the cellulose itself is attacked, leading to damages in strength or other desirable fabric properties. The softness of the cellulosic fabric is a function of residual natural cotton waxes. The non-specific nature of the high temperature strongly alkaline scouring process cannot discriminate between the desirable natural cotton lubricants and the manufacturing introduced lubricants. Furthermore, the conventional scouring process can cause environmental problems due to the highly alkaline effluent from these processes.
The scouring stage prepares the fabric for the optimal response in bleaching. An inadequately scoured fabric will need a higher level of bleach chemical in the subsequent bleaching stages.
Bleaching. The bleaching step decolorizes the natural cotton pigments and removes any residual natural woody cotton trash components not completely removed during ginning, carding or scouring. The main process in use today is an alkaline hydrogen peroxide bleach. In many cases, especially when a very high whiteness is not needed, bleaching can be combined with scouring. The combined process does however require higher dosages of bleach chemicals. The optimal temperature for bleaching is 60.degree. C.-70.degree. C.
In order to minimize quantity of the expensive hydrogen peroxide, adjuncts such as chelators and stabilizers, sodium silicate and surfactants are often employed. As all of these compounds ultimately find their way into the effluent from textiles processes, it is advantageous to minimize their usage.
Enzymatic Treatment of Textiles. The enzyme .alpha.-amylase has been used in the textile industry for the removal of size for many years; indeed, it is one of the earliest known industrial applications of enzymes. Cellulase enzymes have been used in garment finishing applications to mimic the effects of stone washing of denim for the past 8-10 years. The use of the enzyme was rapidly accepted due to the environmental and process benefits. The use of cellulases to bio-polish knits to prevent or inhibit pilling is also known. The enzyme catalase is used in the industry as a milder, more environmentally conscious method to destroy residual hydrogen peroxide in exhausted bleach baths.
Recently, peroxidases and laccases, in combination with mediators are being proposed as a means to decrease the environmental and structural damage caused by the use of chlorine-containing bleaching for some garment finishing applications. Peroxidase enzymes are used in combination with hydrogen peroxide or a source thereof (e.g., a percarbonate, perborate or persulfate). Oxidase enzymes are used in combination with oxygen. Both types of enzymes are used for "solution bleaching", i.e., to prevent transfer of a textile dye from a dyed fabric to another fabric when said fabrics are washed together in a wash liquor, preferably together with an enhancing agent as described in e.g., WO 94/12621 and WO 95/01426. Suitable enzymes for the treatment of textiles include those of plant, bacterial or fungal origin. Chemically or genetically modified mutants are included.
The scouring and bleaching operations employ massive doses of caustic chemicals such as sodium hydroxide and hydrogen peroxide at high temperatures. The cost of these chemicals is substantial, both from the standpoint of initial purchase and environmental burden cost upon disposal of the waste from the operations. The non-selective nature of the process also results in structural damage to the cellulose in the cotton. The impurities in cotton are naturally occurring compounds and as such should be able to be hydrolyzed and removed by enzymes. Various enzymes have been proposed to effect a scouring response. Japanese patent JP 7572747 describes a scouring method for vegetable derived cellulosic fibers, in particular ramie, by using a cellulose decomposing enzyme and a pectin decomposing enzyme. East German patent DD 264947 A1 describes a method to pretreat cotton by using a fungal enzyme complex as desizing agent. The complex may contain fungal cellulase, hemicellulase, pectinase and protease in addition to an amylase derived from fungal, animal, bacterial or vegetable origin. Benefits claimed are an avoidance of alkali and a reduced contamination of waste water. Bach and Schollmeyer (1992) Textilveredlung 27:2-6 describes that the treatment of raw cotton fiber with pectinase and pectinase/cellulase combinations can be bleached to a greater whiteness with hydrogen peroxide than alkaline scoured raw cotton fiber. While the pectinase/cellulase treated and bleached fabric was whiter than the pectinase alone bleached sample, the strength loss was much greater. In contrast, Rossner (Meilland Textilberichte 2/1993, p. 144-148) describes that cotton fabric treated with enzymes and subsequently bleached with hydrogen peroxide cannot be bleached to as great a whiteness as alkaline scoured and bleached fabric. Japanese patent JP 6220772 describes that an enzyme capable of releasing intact pectin from cotton can have a scouring response; the benefits being a milder treatment with reduced energy and lower cost of water disposal without environment pollution. The use of an oil and fat decomposing enzyme either alone or in combination with the pectin liberating enzyme is described in Japanese patent application 6-263524. The benefit of this procedure being the same as those previously described. The harshness of known scouring treatments result in reduced fabric characteristics. Further, the current processes requiring multiple processing steps at different pH and temperature conditions are time consuming and inefficient. Thus, there is a need for an improved scouring process which does not result in a reduction of superior fabric characteristics, as well as a need for more efficient processes.